1. Field of the Invention
The present invention is directed to a support member adapted for positioning and supporting a diffusion tube while the diffusion tube is heated in a furnace as a step in the process of manufacturing semiconductors, integrated circuits and the like. More particularly, the present invention is directed to a support collar capable of supporting a diffusion tube within a furnace and of positioning the diffusion tube relative to heating coils contained in the furnace.
2. Brief Description of the Prior Art
The overall process of manufacturing semiconductors, integrated circuits and like devices includes a step wherein certain intermediate products of the process are heated at approximately 1000.degree.-1200.degree. C. for a 1/4-4 hour time period. This step is commonly referred to in the art as a diffusion step because in this step certain gases usually present in small quantities in an inert carrier gas, diffuse into the material of the semiconductors, integrated circuits and the like. After diffusion and under exposure to heat, the gases decompose to yield such products as elemental boron or arsenic. As is well known in the art, presence of trivalent boron, or pentavalent arsenic in the tetravalent silicon material of the semiconductors is principally responsible for the desirable electrical characteristics of these devices.
During the diffusion or like process the intermediates of the process, commonly referred to in the trade as wafers, are enclosed in a quartz diffusion tube which is heated in a furnace. The diffusion tube usually is slightly longer than the furnace. It is usually supported by support blocks which are located adjacent to oppositely disposed walls of the furnace. Ends of the diffusion tube are located outside of the furnace and are respectively provided with a suitable cap and a suitable gas inlet attachment. The support blocks are usually made of a heat resistant silicon dioxide based material and are commonly referred to in the art as vestibule blocks.
Other steps in the overall process of manufacturing semiconductors integrated circuits and the like which require identical or similar equipment and the use of the diffusion tube in the furnace, include oxidation, polycrystalline and amorphous silicon deposition and the like.
According to present practice in the art, a circular hole or aperture is provided in each of the two oppositely disposed vestibule blocks. The apertures are aligned with one another and are somewhat larger in diameter than the diffusion tube. The diffusion tube is inserted into the furnace through one of the two apertures and is held in position relative to the furnace by the walls surrounding the two apertures. A helically wound heating element or coil is positioned in the furnace to provide and maintain a high temperature therein. A longitudinal axis of the helical heating coil is aligned with the center of the apertures provided in the vestibule blocks. Consequently, when the diffusion tube is operatively positioned within the furnace, it is surrounded by the heating coil although it is not in physical contact therewith.
It is important to position the diffusion tube precisely centered along the longitudinal axis of the heating coil. This is necessary in order to obtain substantially even heating of the entire cylindrical periphery of the diffusion tube and to cause the diffusion and chemical decomposition process to proceed at an even velocity at all points inside the diffusion tube. Furthermore, when the diffusion tube is correctly centered within the helical coil a temperature profile or distribution is obtained along the length and around the periphery of the tube which even if not perfectly uniform, is reproducible. In other words, it is highly desirable that after the diffusion tube is removed from the furnace and is subsequently repositioned therein, the same temperature profile be obtained. This requirement is well appreciated by those skilled in the art. As is described in somewhat more detail below, the state of the art however falls short of achieving the above described objectives.
According to current practice, a heat resistant substantially loose packing material is placed to fill a gap between the diffusion tube and walls of the apertures provided in the vestibule blocks. As the gap is manually filled with the packing material an attempt is made to center the diffusion tube relative to the helical heating coil. The packing material used for this purpose is, of course capable of withstanding the high temperatures used in the furnace. Packing materials currently used for this purpose include rock wool and a high melting glass wool. Both of these materials contain a large percentage of silicon dioxide which principally accounts for their resistance to high temperature. Rock wool is often known in the art under the tradename "K Wool".
The practice of packing rock wool or glass wool around the diffusion tube to support and center the tube relative to the apertures of the vestibule blocks, however suffers from the following serious disadvantages. Manually applying the packing material into the gaps is time consuming and therefore expensive. It is virtually impossible to truly center the diffusion tube relative to the helical heating coil. Furthermore, after the diffusion tube is removed from the furnace for cleaning or other purposes it is virtually impossible to reposition the diffusion tube relative to the helical coil in the same manner as it was previously positioned. It is very difficult to fully close the gap between the diffusion tube and the vestibule block so as to eliminate minor air currents or convections through the packing material. Finally, the packing material readily crumbles and gives rise to dust and airborn particulate matter. This particulate matter and dust has a serious deleterious effect in the diffusion process in that a relatively large number of semiconductors or integrated circuits are often completely ruined by particles which settle on the intermediates during the manufacturing process. Identical or similar problems are present when intermediates of the semiconductors or integrated circuit manufacturing process are subjected to the oxidation, polycrystalline and amorphous silicon deposition and the like steps in the above described furnace.
Thus the practice of supporting and aligning the diffusion tubes relative to the vestibule blocks with the help of loose packing material such as rock or glass wool, results in economically undesirable lost or "down" time in the utilization of the furnace, and in many ruined semiconductors and integrated circuits. The diffusion tube support collars of the present invention are designed to overcome these disadvantages.